The present invention relates to ballasting apparatus of the type generally employed with arc or gas-discharge type lamps and more particularly to such apparatus which protects against excessive arcing and overheating.
As is understood by those skilled in the art, most arc and gas-discharge type lamps require ballasting of one sort or another, typically due to the negative resistance characteristics of the discharges. Further, such lamps typically require the application of relatively high voltages for starting, i.e., voltages which are orders of magnitude greater than the voltages required during continuous operation. As is also understood, these high starting voltages, though momentary, can initiate an arc-over between the lamp leads and ground.
The most commonly used type of ballast apparatus uses a magnetic reactor to limit the lamp current as well as to limit any arc-over current. Magnetic reactors are, for example, typically employed for common fluorescent lamp ballasts. Mass produced, these ballasts are relatively inexpensive, but are quite heavy and are a source of heat. Further, if a short circuit occurs, the ballasts can overheat and bake, eventually failing entirely.
Magnetic reactance type ballasts have also been used with many other types of lamps. However, as lamp power goes up, so does the size and weight of the magnetic ballast. Further, with more powerful lamps, e.g., sodium vapor lamps, higher starting voltages are needed and extra insulation or air-space must be used around the wires, connectors and sockets to avoid the creation of an arc to the chassis or safety ground. Conventionally, arc-over must be avoided for two reasons. One, if the high voltage is not impressed on the lamp long enough, the lamp may not start. Secondly, the current in an arc-over can reach hundreds of amperes very quickly. It is thus essential to limit arc-over currents to a safe level in order to avoid a fire hazard. A circuit breaker or fuse may be a minimum acceptable protection but is only acceptable if the user does not mind resetting the breaker or replacing the fuse. A large magnetic reactor in series with the lamp's power leads will limit current in one sense but will also tend to maintain rather than cut-off the current during arcing.
More recently, so-called switcher type ballasts have been devised to alleviate the weight and overheating problems experienced with magnetic ballast used with high-power lamps. However, since the switcher type ballast does not conventionally employ a large series reactance to limit lamp current, an arc-over can quickly create destructively high currents. To avoid such currents, it has been a common practice to incorporate an isolation transformer which can block arc-over currents to ground but can pass the normal lamp current. Such isolation transformers may be of the line frequency type, i.e., located at the input of the ballast or a high frequency type which is within the ballast itself. An input transformer must pass the full lamp power at low frequency. Such transformers tend to be large and heavy. The high frequency types may be made smaller and lighter than line frequency transformers but are expensive and still are large in comparison with the typical high frequency transformers since the characteristics of the lamps require both high voltage and high current, resulting in a need for a lot of large wire in the transformer structure.
Among the several objects of the present invention may be noted the provision of novel ballasting apparatus for discharge lamps; the provision of such apparatus which controls arc-over currents; the provision of such apparatus which controls lamp power; the provision of such apparatus which does not require the use of an isolation transformer; the provision of such apparatus which minimizes heating; the provision of such apparatus which is highly efficient and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.